1. Field of the Invention
The present invention relates to high reflectance coatings. More specifically, it pertains to a durable silver thin film high reflectance coating for diffraction gratings.
2. Description of Related Art
Diffraction gratings are used in many optical applications, including spectroscopic instruments and in chirped-pulse amplified lasers (CPA). By way of example, in the design of short pulse laser systems amplification, using the CPA technique, high energy, short pulses are typically made possible by first stretching the pulse in time, amplifying it, and then recompressing it temporally via diffraction gratings. Such short-pulse laser designs conventionally require a total of four bounces off of a system-incorporated single grating or a pair of gratings in the pulse compressor and stretcher stages. The throughput in the pulse compressor stage alone is thus proportional to the fourth power of the grating efficiency. A modest 4% increase in the diffraction efficiency of the gratings from 90 to 94% can, for example, improve throughput of the compressor stage by 20%, which results in a significant increase in laser energy to the target. While researchers have developed multi-layer dielectric coatings (MLD) to overcome the efficiency and power handling limitations of metallic gratings, such MLD gratings have limited bandwidth for very short pulse (e.g., less than 50 femtosecond) laser systems that require greater than 100 nm of band-pass.
The standard coating for metal gratings for CPA lasers is gold. Aluminum is a common overcoat for gratings for UV-visible spectroscopic applications. Silver, however, has the highest reflectivity, lowest polarization splitting, and lowest absorption from 400 nm through the infrared of all of the metals. However, silver is susceptible to attack by oxygen and constituents of atmospheric pollution, such as chlorine, sulfur, and ozone. When such substances react with the reflective coating, the silver layer becomes tarnished so that the required optical properties of that layer are lost. Specifically, the tarnish lowers the effective reflectivity of the silver coating.
An approach for applying durable silver coatings to substrates is disclosed and claimed in U.S. Pat. No. 6,078,425, titled “Durable Silver Coating For Mirrors”, to Wolfe, et al, and is herein incorporated by reference in its entirety. Such a silver coating developed by Wolfe does not tarnish and can be used in harsh environments, which improves performance over traditional metallic reflectors when coated on a planar surface.
Wolfe et al., have also designed durable thin film coatings that permit light transmission in the visible range while reflecting infrared radiation. U.S. Pat. Nos. 5,377,045 and 5,521,765 to Wolfe et al. disclose thin film designs having a first layer of oxide, a layer of nickel chromium alloy, a silver layer, another layer of nickel chromium alloy, and a top layer of silicon nitride. U.S. Pat. No. 5,563,734 to Wolfe discloses silver layers sandwiched between layers of nickel-chromium nitride and silicon nitride with a further oxide outer layer. These thin films are used as filters, or substrates that are transparent to visible light, but block out infrared radiation.
Efforts on recent improvements of gratings are described in “High-efficiency metallic diffraction gratings for laser applications,” by Boyd et al., Applied Optics, Vol. 34 (10), 1697–1706, “Universal grating design for pulse stretching and compression in the 800–1100 nm range,” by Britten et al., Optics letters, Vol. 21 (7), 540–542, 1996, and in U. S. Pat. No. 5,907,436, titled, “Multi-layer dielectric Diffraction Gratings,” issued May 25, 1999 to Perry et al.
Accordingly, a need exists for a durable metallic coating on diffraction grating surfaces so as to increase bandwidth for laser and spectroscopy systems that utilize such grating components. The present invention is directed to such a need.